A gasoline engine in an automobile generally has a fuel injection valve provided at an intake pipe, and fuel injected from the fuel injection valve is supplied to an intake port. However, during cold starting with no sufficient warm-up, part of the fuel injected from the fuel injection valve tends to stick to the inner wall surface of the intake port or the surface of the intake valve and fails to enter the combustion chamber. This substantially reduces the injection amount. In order to secure an air-fuel ratio equivalent to that in a sufficiently warmed-up state, the injection amount is often corrected by adding fuel in such a case.
The amount of fuel thus sticking, for example, to the inner wall surface of the intake port, without contributing to combustion varies depending on the nature of the fuel, especially the level of its volatility. Fuel nature varies among the manufacturers, the seasons, and the distribution areas even if the fuel is of the same kind. Therefore, fuel nature must be measured highly precisely in order to accurately correct the injection amount.
A known technique for measuring fuel nature takes advantage of the characteristic that the dielectric constant of fuel changes depending on the fuel nature. According to this technique, a capacitor-type detector is provided and determines whether the fuel is light gasoline or heavy gasoline based on a capacitance of the detector corresponding to the dielectric constant of the fuel (see Japanese Utility Model Laid-Open Publication No. Hei 4-8956). According to this technique, an oscillation circuit that generates a signal at a frequency corresponding to capacitance is provided to obtain the capacitance. Another known technique takes advantage of the characteristic that the refractive index, boiling point, and molecular heat of a fuel changes depending on the fuel nature (see Japanese Patent Laid-Open Publication No. Hei 4-1438). According to the disclosure of Japanese Patent Laid-Open Publication No. Hei 4-1438, an optical fiber is immersed in the fuel, and the quantity of light passed through the optical fiber is analyzed to obtain the refractive index.
In order to obtain the volatility of fuel based on the dielectric constant and the refractive index, a relation between the dielectric constant and refractive index of the fuel and the volatility of the fuel must be previously known. However, the relationship varies among the manufacturers of the fuel, the seasons, and the distribution areas and it is not necessarily easy to acquire accurate information between them.